Odd issue with two SNATed Firewalls and Wireless router

[Andrew Gargan]

Hi All

have an issue with a network setup. 

the issue is this some client machines on my network (10 of +- 150) keep
losing their connection to our mail server or any other mail server we
set up their accounts on.

This is an issue with my network because as soon as they are on dialup
or iburst or ADSL elsewhere they don't have this issue.

I have a 10.0.0.0/8 network which is my internal net.
this net routes traffic through 10.0.2.1 (SNAT) which is connected to my
DMZ 172.16.0.0/16.
the traffic is then sent out to the world via 172.16.0.1(SNAT) over
sentech mywireless.

I have isolated the issue to being on the 172.16.0.1 machine since a
ISDN leased line in the DMZ works 100%.

Has anyone experienced similar issues using a shared NATed mywireless
.... 

most of the mail comes down .... it seems to break when transmissions
are over +-600 KB)

I was told that changing the MTU for the ppp0 device to 1300 would help
but no luck there.

Also one strange thing I noticed (though I don't know TCP/IP that well)

was the txqueuelength value of 3 for the ppp0 interface

here is my kernel: 2.6.9-1.667 (FC3)

here is ifconfig:
eth0      Link encap:Ethernet  HWaddr 00:03:47:71:7B:36
          inet addr:172.16.0.1  Bcast:172.16.0.255  Mask:255.255.255.0
          inet6 addr: fe80::203:47ff:fe71:7b36/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:27879849 errors:0 dropped:0 overruns:0 frame:0
          TX packets:26767743 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:669378689 (638.3 MiB)  TX bytes:3461641354 (3.2 GiB)

eth1      Link encap:Ethernet  HWaddr 00:03:47:71:7B:37
          inet addr:10.0.7.2  Bcast:10.255.255.255  Mask:255.0.0.0
          inet6 addr: fe80::203:47ff:fe71:7b37/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:27333550 errors:0 dropped:0 overruns:0 frame:0
          TX packets:28013971 errors:1 dropped:0 overruns:0 carrier:1
          collisions:614337 txqueuelen:1000
          RX bytes:3798771770 (3.5 GiB)  TX bytes:845067479 (805.9 MiB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:13651 errors:0 dropped:0 overruns:0 frame:0
          TX packets:13651 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:556916 (543.8 KiB)  TX bytes:556916 (543.8 KiB)

ppp0      Link encap:Point-to-Point Protocol
          inet addr:196.35.170.61  P-t-P:66.18.87.50 
Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1400  Metric:1
          RX packets:7595399 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7622079 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:1453866192 (1.3 GiB)  TX bytes:2513716398 (2.3 GiB)


and iptables -L:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
DROP       tcp  --  anywhere             anywhere            tcp
flags:SYN,RST,ACK/SYN
DROP       tcp  --  anywhere             anywhere            tcp
flags:SYN,RST,ACK/SYN

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
TCPMSS     tcp  --  anywhere             anywhere            tcp
flags:SYN,RST/SYN TCPMSS clamp to PMTU
ACCEPT     all  --  anywhere             anywhere            state
RELATED,ESTABLISHED

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination


and mii-tool:
eth0: negotiated 100baseTx-FD, link ok
eth1: no autonegotiation, 10baseT-HD, link ok

eth1 is the ether used to connect to the mywireless ... for routing
purposes the ip it has assigned isn't really used ... 

I am using rp-pppoe I think ... 

Andrew Gargan
Developer
Interface Media (PTY) Ltd.
Tel: 011 507 3003